1. Field of the Invention
The present invention concerns a device for non-contacting transmission of electrical signals between a part moving relative to another part, of the type wherein at least one strip conductor pair for symmetrical signal transmission is applied on a first of the two parts (in which strip conductor pair the electrical signals are fed from a transmission module for differential signal transmission) and in which at least one reception element is attached on a second of the two parts, the reception element moving during the relative movement of the two parts at a slight separation along at least one segment of the strip conductor pair, and being connected with a reception module. Such a device is suitable for the non-contacting transfer of data between the rotating part and the stationary part of a computed tomography apparatus.
2. Description of the Prior Art
In the operation of a computed tomography apparatus, the data acquired by the x-ray detectors must be transferred from the rotating part to the stationary part of the system in order to be further processed. The data quantity to be transferred per time unit constantly increases with the further development of computed tomography systems, particularly computed tomography systems of the third generation. The devices that are available for data transfer (such as the known slip rings operating without contact) are limited in terms of the transfer rate. A need therefore exists to use a number of these slip rings in parallel (next to one another) in order to achieve an increased transfer rate by a simultaneous data transfer in real time.
In the non-contacting transmission of electrical signals using slip rings, use is made of the electrical field that arises given the feed of electrical signals at the slip ring. Through (normally capacitive) coupling, these signals can be coupled into a reception element past which the slip ring moves at a slight distance. This reception element is arranged at the stationary part of the computed tomography apparatus and is connected with a reception module for decoding the received signals. The slip ring is normally fashioned as a strip conductor in which the electrical field modulated with the fed electrical signals can propagate. Such a strip conductor, however, also leads to unwanted electromagnetic radiation that is called electromagnetic perturbing radiation. This perturbing radiation accumulates given the usage of a number of strip conductors, as can be necessary for a high data transfer rate. This can lead to the situation that the required electromagnetic compatibility (EMC) for medical apparatuses can no longer be maintained. The perturbing radiation already varies very significantly due to manufacturing tolerances from component to component and additionally depends on the spectral composition of the transmitted signals.
To reduce the electromagnetic perturbing radiation given non-contacting data transfer in computed tomography systems, it is known (for example from EP 1 051 816 B1) to significantly spectrally spread the signals to be transmitted in order to reduce the radiated perturbation level by decreasing the average spectral power density. For serial high-speed data transfer, however, this technique cannot be used since the requirements for the lack of jitter of the components used are too high with higher data rates.
Shielding of the strip conductor is proposed in U.S. Pat. No. 5,530,425, but does not operate efficiently enough at high data rates in order to justify the complexity and the resulting costs required for such shielding.
A further known technique for reduction of electromagnetic perturbing radiation is the use of a strip conductor pair through which the electrical signals are differentially transmitted. The coupling that occurs between two strip conductors lying close to one another is utilized. Such a strip conductor pair enables the propagation of directed electromagnetic waves in a push-pull mode (differential mode) or in common mode (push-push mode). In differential signal transmission, the signals to be transmitted are coupled into both strip conductors of the strip conductor pair offset by 180° in terms of phase. This leads to a difference voltage between the two strip conductors that correspond to twice the signal amplitude. In the common mode signal transmission, the signal is coupled into both strip conductors with the same phase such that no voltage difference exists between the two strip conductors.
Differential signal transmission has the advantage that, in the ideal case, the electromagnetic waves radiated by the two strip conductors of the strip conductor pair mutually compensate in the far field, and thus no perturbing radiation, or only very slight perturbing radiation, occurs. Differential signal transmission via strip conductor pair is therefore utilized in U.S. Pat. No. 5,530,422 in order to achieve a non-contacting signal transmission between the rotating part and the stationary part of a computed tomography apparatus with reduced perturbing radiation. The differential impedance of a symmetrical strip conductor pair corresponds to twice the impedance of the individual strip conductor minus the impedance that results due to the electromagnetic coupling of the two strip conductors. A strong coupling due to a close arrangement of both strip conductors reduces the differential impedance, but increases the common mode impedance. Under the opposite conditioning, the differential impedance approaches the sum of both individual impedances of the strip conductors when only a very weak coupling of both strip conductors exists. In U.S. Pat. No. 5,530,422 (already cited), a parallel termination is employed for both strip conductors. This termination can be a good termination for differential signal transmission, but not for common mode signals.
A problem in the usage of the technique of differential signal transmission is that no ideal compensation of the radiated electromagnetic fields is achieved in the far field due to tolerances of the components of the transmission module, or due to different conductor lengths, or material inhomogeneities of both strip conductors of the strip conductor pair. Rather, due to such tolerances the electromagnetic perturbing radiation can assume high values, which are always undesirable.